1. Field of the Invention
The present invention relates generally to a method and assembly for sealing a draw furnace, and more particularly, to a sealing assembly and method for sealing the bottom of an optical waveguide draw furnace.
2. Technical Background
Relatively high temperature heat sources are required for drawing high strength, low loss optical waveguide fibers from a high silica-content optical waveguide fiber preforms or blanks. Two of the predominant heat sources utilized for drawing such fibers are zirconia furnaces and graphite furnaces. Fiber draw furnaces generally operate at temperatures greater than about 1900.degree. C., typically as high as about 2050.degree. C.
A disadvantage associated with zirconia induction furnaces is that extended use and thermomechanical stresses due to poor thermal shock resistance cause cracks in the muffle and susceptor. This cracking causes zirconia particles to migrate from the inner surface of the furnace onto the preform and/or fiber being drawn from the preform, resulting in substantially weakened fiber and unacceptable product losses. Moreover, zirconia induction furnaces are sensitive to rapid changes in temperature. Accordingly, significant amounts of time are necessary for increasing and decreasing the temperatures within the furnace. Rapid heating and cooling of the furnace results in fracturing of the zirconia muffle, which necessitates replacement of the muffle and results in significant furnace down time.
Such shortcomings lead to the development of graphite induction furnaces. Graphite induction furnaces typically include a graphite muffle which is insensitive to rapid changes in termperature and thus is less susceptible to cracking. It has been found, however, that graphite furnaces are susceptible to oxidation at temperatures approaching about 450.degree. C. and that oxidation is particulary problematic when the graphite is exposed to the high temperatures used to draw optical waveguide fibers. For this reason, among others, it is preferrable to maintain an inert atmosphere within the draw furnace. Oxidation occurs when gasses from ambient atmosphere react with the solid carbon muffle at high temperatures according to the following reactions:
(1) C+O.sub.2.fwdarw.CO.sub.2 ; and
(2) C+CO.sub.2.fwdarw.2CO.
A typical onset temperature for reaction (1) for a graphite grade used in a draw furnace is about 700.degree. C. Reaction (2) becomes significant above 900.degree. C. These reactions of the furnace muffle with oxygen and carbon dioxide cause the furnace muffle to be consumed, especially at elevated fiber drawing temperatures, and are referred to generally as CO events.
The graphite muffle material is a composite of graphite grains bonded together by a carbon binder matrix. It is believed that the binder material is more susceptible to oxidation than the graphite grains. Therefore, when the composite of the two materials is exposed to air, and thus oxygen at temperatures above the oxidation onset temperatures, the matrix binder material preferentially oxidizes. The graphite grains, having no binder left to hold them in place, are then free to fall away from the composite structure. It is believed that this mechanism causes graphite particulate to migrate from the muffle wall to the fiber preform and/or fiber during drawing.
Graphite particulate that becomes incorporated into the fiber during drawing causes unacceptable product losses due to point defects. Point defects manifest themselves as sharp attenuation increases in the signal transmitted through the fiber. Point defect product losses due to graphite particulate from a draw furnace can be greater than about 5%, which is an unacceptably high loss. Graphite particulate that has adhered to the fiber during the draw process also contributes to fiber breaks.
In order to reduce graphite particulation produced by oxidation of the graphite muffle material, and thus the number of resulting point defects, an inert gas is typically supplied to the interior of the furnace to prevent ambient air and other gases from entering the furnace. Unfortunately, there are a number of operations that occur during optical waveguide fiber draw, which present ambient air and other gases with the opportunity to enter the furnace despite delivery of the highest inert gas flow rates into the furnace. When a mistake is made during these draw operations, either by human error, or by mechanical failure, ambient air and/or other gases are often permitted to enter the furnace and a CO event occurs.
Many of these CO events are caused during movement or operation of the bottom door assemblies presently used to close the opening in the bottom of a draw furnace during blank load, unload and idle periods. Typical bottom door assemblies include a single gasket that is visually inspected to determine if a proper seal has been established when the bottom door assembly is brought into engagement with the bottom of the furnace during any of these periods. The purpose of the seal is to prevent the entrance of air into the furnace. Often, excess optical fiber depending from the fiber blank after a fiber break, or other foreign debris is trapped between the gasket and the bottom of the furnace when the bottom door assembly is closed. This in turn produces a gap which prevents a proper seal. Moreover, deflection of the bottom door assembly due to mechanical stresses applied to the various components of the assembly through repeated and continuous use will also result in a faulty seal. Unfortunately, this is often overlooked during visual inspection of the seal. If the top seal is removed from the draw furnace under such conditions, or during blank unload, air is rapidly sucked up into the furnace through any gap in the seal. Likewise, if the operator forgets to close the bottorm door assembly or improperly seats the bottom door assembly and removes the top hat, air is rapidly introduced into the furnace. This phenomenon, commonly referred to as the chimney effect, can result in a number of CO events.
In view of these and other shortcomings, an improved assembly and method of sealing an optical waveguide draw furnace is needed, which prevents the entrance of air and other unwanted gases into the draw furnace.